Superheterodyne receiver



Nov. 12, 1935. D. GRlMEs SUPERHETERODYNE RECEIVER Filed March 21, 1952/D INVENTOR Il* DAv|o GRM' BY /f www ArroRNEY Patented Nov. 12, 1935UNITED STATES ATENT OFFICE SUPERHETERODZNE RECEEVER Application March21, 1932, Seria-l No. 600,124

9 Claims.

My present invention relates to superheterodyne receiving systems, andmore particularly to an improved type of frequency changer, or firstdetector, circuit for use in a superheterodyne receiver.

It has been well appreciated for some time that power detectors aredesirable, if not necessary, where the desired station is of sufficientstrength, either by virtue of strong transmission or considerableamplification in the receiving set. Thus, in a tuned radiofrequencyreceiver employing modern radio frequency amplification, such asproduced by a multi-stage radio frequency amplilier, a detector must beemployed which will not readily overload. This is so since a detectorcircuit which readily overloads constitutes the weak link in theamplification chain, and Will result in premature distortion. Similarly,the same condition pertains in the second detector of a superheterodynereceiver Where both the radio frequency and intermediate frequencyamplification have created a signal of considerable strength.

The need for a power detector in the first detector stage of thesuperheterodyne receiver is not so apparent and has not beenappreciated. Obviously, the desired signal strength at this point isvery weak as the signal has been subjected to little or noamplification. Of course, the entire available grid swing determined bythe permanent grid bias on the first detector tube is not available forthe incoming signal because of the presence of the induced localoscillator swing. However, the amount of local oscillator peak swing hasbeen maintained at a point sufliciently below the maximum permanent biason the rst detector to allow the incoming signal to be superimposedthereon without causing the instantaneous peak swing in the firstdetector grid to run into the positive region at any time as this causesdistortion.

The aforegoing discussion relates only to the reception of the desiredsignal. Now, there is a real defect which arises from other broadcastchannels than the desired one. It often happens that the strength of thesignal from an adjacent undesired channel on the grid of the firstdetector is greater than the strength of signal from the desiredchannel. This arises by virtue of the limited amount of tuned radiofrequency selectivity preceding the rst detector. The presence of thisexcessive undesired signal is of itself no detriment, and will cause noundesired responses in the receiver by virtue of its mere presencebecause the frequency differential between the same (Cl. Z50-20) and thelocal oscillator is not the frequency of the intermediate tunedcircuits.

However, if the effect of this excessive local undesired station on thegrid of the rst detector is great enough to cause the instantaneous grid5 swing of the first detector to run into the positive region then apeculiar type of distortion and interference takes place in the form ofcrossmodulation on adjacent desired channels and multiple responses. Thelatter occur Whether or 10 not a desired channel is being received.

There are three solutions for such defects', and such solutions are asfollows:

1) Increased radio frequency selectivity preceding the rst detector maybe employed; l5 (2) The local oscillator induced voltage may be reduced;

(3) A power detector employing l ample grid bias or other types ofdetectors which are not subject to overload, may be utilized in thefirst detector stage.

The first expedient mentioned above is costly as it necessitatesadditional tuningv condensers with the associated diiculties of gangingthese additional condensers. Furthermore, additional tuning stagesrequired extra precautions in shielding to prevent the undesiredstations from reaching the first detector other than through theorthodox path; The secon-d expedient involving reduced local oscillatorswing greatly reduces the translation efciency of the rst detector.

Accordingly, it may be stated that the third solution referred to abovecomprises the basis of the present invention, and offers as only realsolution to an aggravating problem. Hence, it may be pointed out that itis one of the main objects of the present invention to provide in therst detector circuitof a superheterodyne receiver an electron dischargetube arrangement so designed that its grid circuit can handle theinduced local oscillator voltage swing, the desired channel signalswing, and the maximum amount of undesired signal swing possible toencounter in practice without resulting in an instantaneous grid swingsufliciently great to pass into the positive region of the firstdetector characteristic.

Another important object of the present invention is to provide asuperheterodyne receiver employing but a single tuned stage of radiofrequency amplification prior to the rst detector, and to operate therst detector with such plate and grid voltages that the maximum gridvoltage swing does not overload the detector tube.

Another object of the present invention is to minimize cross modulationbetween a desired signal and an undesired adjacent channel signal in asuperheterodyne receiver by designing the first detector of thesuperheterodyne receiver in such a manner that. the instantaneous gridswing of the latter is never great enough to run into the positiveregion of the detector characteristic.

And still other objects of the present invention are to improvegenerally the operating efficiency of superheterodyne receivers, and toespecially design the first detector circuit of a superheterodynereceiver as a power detector in such a manner that the first detectorcircuit is not only reliable in operation, but economically manufacturedand installed.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically one circuit organization whereby my invention may becarried into effect. v f

Referring now to the accompanyingdrawing, in which there is shown aconventional type of superheterodyne receiver, it will be noted that theVusual grounded antenna A, G is employed for collecting intelligencemodulated carrier frequency energy, within the so-called broadcast rangebetween 550 to 1500 kilocycles. The collected carrier energy isimpressed as. at M1, upon the tuned input circuit of a radio frequencyampliiier l, it being noted that the input circuit of the amplifier isadapted to be tuned through the frequency range of the receiver by avariable condenser 2. The amplified radio frequency energy is thenimpressed upon the input circuit of the first detector, which includesan electronic discharge tube 3 of the screen grid type. The inputcircuit of the tube 3 includes the variable condenser 4 which is adaptedto tune the input circuit of the first detector in a manner similar tothe tuning of the amplifier I.

A local oscillator 5 is shown in conventional form, and is adapted to betuned by a variable condenser 6 through a frequency range which differsfrom the frequency range through which the condensers 2 and 4 areoperative by a desired intermediate frequency. The locally producedenergy is impressed, as at M3, upon the grid circuit of the firstdetector tube 3. Those skilled in the art are well acquainted with thephenomenon occurring in the first detector, or frequency changercircuit, and it may be briefly stated that they energy impressed uponthe input circuit of the tube 'through the coupling M2, and the 1ocallyproduced energy impressed uponthe grid circuit of tube 3 through thecoupling M3, heterodyne to produce in the output circuit of the firstdetectorv the Vdesired intermediate frequency. The intermediatefrequency-energy is transmitted through a coupling M4 to the inputcircuit of the intermediate frequency amplifier 1, the input circuit ofthe latter being maintained fixedly tuned by a condenser 8 to thedesired intermediate frequency. While the amplifier l has beenconventionally represented, it will be understood that the amplifier mayconsist of one or more stages of intermediate frequency amplification,and the design thereof is so well known to those skilled in the art thatit need not be described in any further detail in the presentapplication.

The amplified intermediate frequency energy is then transmitted throughthe coupling M5 and the desired channel.

audio frequency amplification in the well known 5 and usual manner. Theamplied audio frequency energy is then utilized in any well knownfashion, as by head phones, loud speaker or any other type ofreproducer.

As pointed out heretofore it has been well ap- 10 'preciated for sometime that power detectors are desirable, and in fact necessary, wherethe desired signal energy is of suicient strength due to considerableamplification in the receiving set. Thus, a detector must be employedwhich will not readily overload, as such a condition will constitute theweak link in the amplification chain and will result in prematuredistortion. Accordingly, the second detector l5 may be a power detectorsince` both the radio frequency and intermediate frequency amplificationhave created a signal of considerable strength.

It is not believed necessary to describe in any further detail thecharacteristics of the design of the second detector as a power detectorsince Y this is clearly disclosed by W. L. Carlson in United StatesPatent 1,770,838 of July 15, 1930. It is to be noted that if thedetector lil is a power detector a single stage of audio frequencyamplification is sufficient to operate a loud speaker.

Considering, now, the subject matter of the present invention it isagain pointed out that the need for a power detector in the firstdetector stage of the superheterodyne receiver has not been appreciated.Obviously, the desired signal strength at the first detector point isvery weak since it has been subjected to but one stage of radiofrequency amplification. It often happens that the strength of signalfrom an adjacent undesired channel on the grid of the first detectortube 3 is greater than the strength of signal from This arises from thelimited amount of tuned radio frequency selectivity ahead of the rstdetector.

If the effect of this excessive local undesired station, combined withthe local oscillator swing and the desired station, on the grid of thefirst detector is 'great enough to cause the instantaneous grid swing ofthe tube 3 to run into the positive region, then it can be readilydemonstrated that a peculiar type of distortion and interference takesplace in the form of cross-modulation on adjacent Ydesiredchannels andmultiple responses. The latter may occur whether or not a desiredchannel is being received.

It will be appreciated that such cross-modulation, or cross talk, isparticularly annoying in the case of a superheterodyne receiver.y Thelatter is considered one of the most selective types of radio receivers,and hence the existence of cross-talk and multiple responses wouldobviously cause the operator of the superheterodyne receiverconsiderable disappointment in the selective qualities of the receiver.As explained above, the present invention offers the only real solutionto this aggravating problem.

By employing a power detector in the first detector circuit which isoperated on high plate voltages such as 250 volts with an associatedincreased negative grid bias of approximately thirty to thirty-fivevolts, the grid circuit can handle the induced oscillator voltage swing,the desired channel signal swing, and the maximum amount of undesiredsignal swing possible to encounter in practice without resulting in aninstantaneous 76 grid swing sufficiently great to pass into the positiveregion. The source B conventionally designates the 250 volt source, thelatter being shunted by a radio frequency by-pass condenser li, and thesource C conventionally designates the grid bias source capable ofapplying approximately QG to 35 volts to the grid of the tube 3.

By means of the arrangement described the problem of eliminating thecross modulation distortion is simply and effectively solved. Thisinvention is particularly necessary in superheterodyne receiver circuitsemploying automatic volume control because the radio frequency gainpreceding the first detector is automatically increased in-proportion asthe desired signal is weak. This increase in radio frequencyamplification thus tends to bring in the undesired adjacent channel toan excessive degree sufficient to run the instantaneous grid swing ofthe first detector into the positive region. Hence, when utilizing anautomatic gain control circuit, as of the type disclosed by H. T. Friisin United States Patent 1,675,848 of July 3, 1928, it is extremelybeneficial to design the first detector circuit to operate as a powerdetector. It is not believed necessary to go into the details ofconstruction of an automatic volume control circuit since the latterpatent clearly discloses the design in operation of such a circuit in asuperheterodyne receiver, and additionally since those skilled in theart are acquainted with the manner of embodying an automatic gaincontrol circuit in a radio receiver of the superheterodyne type. Therehas been shown in the drawing for this reason a conventionalrepresentation designated AVC. This is to be understood as representingan automatic amplification control circuit of the type shown in theFriis patent.

The operation of the present invention is believed to be clear from theaforegoing description. It is, of course, understood that the tuningcondensers 2, 4 and 6 are mechanically coupled as shown by the dottedlines I2, for uni-control tuning. In order to maintain the desiredintermediate frequency difference between the local oscillator circuitand the radio frequency amplifier' and first detector circuits aconstant quantity throughout the operating frequency range, anarrangement is employed which is disclosed by W. L. Carlson in U. S.Patent 1,740,331 of December 1'?, 1929. Such a constant intermediatefrequency difference arrangement is too well known to those skilled inthe art to require any additional description in the presentapplication.

While the first detector circuit has been shown to embody a powerdetector tube 3 employing sufficient grid bias to prevent overload atmaximum instantaneous grid swing, it is to be clearly understood thatthe present invention is not limited to a power detector operating bythis manner in virtue of increased grid bias and plate voltage. Anyother type of detector which is not subject to overload may be employedin the first detector circuit, since the essential principle underlyingthe present invention involves utilization of a first detector circuitwhich is designed in such a manner that maximum instantaneous grid swingnever passes into the positive region of the detector tubecharacteristic.

Additionally, it is pointed out that a type of electron discharge tubeknown as the variable mu tube may be employed for the tube '3 in thefirst detector circuit. Such a tube and its characteristics has beendescribed in the Proceedings of the Institute of Radio Enkineers involume 18,

1930, the paper being entitled Reduction of Distortion, attention beingparticularly directed to pages 2122 and 2123 of this article. Thevariable mu tube employs high plate voltages and increased grid biasreferred to herein, and provides Q5 a tube design which lends itselfreadily to the purposes sought to be attained in the present invention.

While I have indicated and described one arrangement for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as l5` set forth in theappended claims.

What I claim is:

l. A superheterodyne receiving circuit comprising a single stage oftuned radio frequency amplification, a grid biased tube detectoroperating as a first detector, an intermediate frequency amplifier, anda second detector all arranged in cascade, the selectivity of thereceiving circuit network preceding the said first detector beingsufficiently low to permit the impression upon the first detector gridof an undesired adjacent channel signal of substantial intensity, meansto impress a high negative biasing voltage on the grid of the firstdetector tube, a source of high positive plate voltage therefor, thegrid and plate voltages being sufficiently high to give a detectorcharacteristic such that the grid circuit of the first detector canhandle a desired channel signal swing and the maximum amount ofundesired signal swing possible to encounter in practice withoutresulting in an instantaneous grid swing sufficiently great to pass intothe positive region of the said detector characteristic.

2. A superheterodyne receiver comprising a single stage of tuned radiofrequency amplification, a local oscillator, a first detector tubecircuit,

an intermediate frequency amplifier, and a second detector circuit,means to impress a negative biasing voltage on the grid of the firstdetector tube of the order of at least 30 to 35 volts, 45 theselectivity of said single stage preceding the said first detector beinginsufiicient to reduce the strength of signal from an adjacent undesiredchannel on the grid of the first detector with respect to the strengthof signal from the desired channel, and a source of commensurately highpositive plate voltage for the first detector tube, whereby the gridcircuit of the first detector can handle the induced oscillator voltageswing, the desired channel swing, and the maximum amount of undesiredsignal swing possible without result,- ing in an instantaneous gridswing sufficiently great to pass into the positive region of the firstdetector characteristic.

3. A superheterodyne receiving circuit comprising a radio frequencyamplifier, a first detector circuit, a local oscillator circuit, anintermediate frequency amplifier, a second detector circuit, theselectivity of the radio amplifier being sufiiciently low to permit thetransmission to the first detector of an undesired adjacent channelsignal having an intensity comparable to that of the desired signalintensity, means to impress a negative biasing voltage on the grid ofthe first detector tube of the order of 30 volts, and a 70 source ofpositive plate voltage for said first detector which is so high that themaximum instantaneous grid swing of the first detector tube is notcapable of running into the positive region of the first detectorcharacteristic.

4. In the operation of a superheterodyne radio receiver including anintermediate frequency amplifier, a first detector tube, and a secondNdetector tube, the method Which includes reducing the selectivitypreceding the first detector to an amount such that the strength ofsignal from and adjacent undesired channel on the grid of the firstdetector is comparable with or greater than the strength of signal fromthe desired channel, obtaining rectification in the first detector tubeby negatively biasing the grid thereof, and increasing the negative gridbias and positive anode potentials applied to the first detector tube intion on adjacent desired channels and multiple responses caused byoverloading of the first detector tube, which includes so adjusting theconstants of the first detector that the maximum instantaneous gridswing of the latter never runs into the positive region of the firstdetector characteristic.

6. A superheterodyne receiver including only a single stage of tunedradio frequency amplification prior to the first detector circuit, theselectivity ahead of the rst detector being insufflcient tosubstantially suppress undesired adjacent channel signals, characterizedby the feature that the constants of the rst detector circuit are soadjusted that the maximum instantaneous grid swing of the first detectoris prevented from running into the positive region of Kthe firstdetector characteristic.

7. In a superheterodyne receiver comprising a radio frequency amplifieradapted to have its amplification automatically varied to maintain thereceiver volume at a predetermined level regardless of carrierfluctuations, a local oscillator, a frequency changer tube circuitcoupled to said amplifier and oscillator for producing a desired yaoeosabeat frequency, the said amplifier having insufficient selectivity toreduce the undesired adjacent channel signal intensity whereby theundesired signal is substantially amplified when weak desired signalsare received, the constants of the frequency changer tube being soadjusted as to prevent the instantaneous grid swing of .the frequencychanger tube from running into the positive region of itscharacteristic.

8. In a superheterodyne receiver comprising a I radio frequencyamplifier adapted to have its. amplificationautomatically Yvaried tomaintain the receiver volume at aV predetermined level regardless ofcarrier fiuctuations, a local oscillator, a frequency changer tubecircuit coupledl to said amplifier and oscillator for producing adesired beat frequency, the said amplifier having insuflicientselectivity to reduce the undesired adjacent channel signal intensitywhereby the undesired signal is substantially amplified When i Weakdesired signals are received, the constants of the frequency changertube being so adjusted as to prevent the instantaneous grid swing of thefrequency changerV tube from running into the positive region of itscharacteristic, means for: amplifying said beat frequency, and adetector circuit coupled to said beat frequency amplifying means, saidsecond detector comprising a power detector capable of deliveringsufficient power tooperate a reprcducer with but a single z stage 0faudio frequency amplification.

9. A method of receiving signals with a superheterodyne receiver, of thetype including a signal collector, a first frequency changer tube, a

Vsecond frequency changer tube and a repro-g ducer, which consists incollecting broadcast signals, reducing the selectivity between thecollector and first frequency changer tube to a point such thatundesired signals are applied to the latter with an intensity comparableto the intensity of desired signals whereby reproduction of demodulateddesired signals is accompanied by reproduction of demodulated undesiredsignals, and maintaining the relative grid and plate potentials of thefirst frequency changer Ytubej such that overloading of the latter byapplied collected signals is constantly prevented and the effect of saidreduced selectivity substantially compensated for.

DAVID GRIMES. I

